Precisely controlled fiber deposition by electrostatic fields

ABSTRACT

Applications of electrospinning (ES) range from fabrication of biomedical devices and tissue regeneration scaffolds to light manipulation and energy conversion, and even to deposition of materials that act as growth platforms for nanoscale catalysis. One major limitation to wide adoption of electrospun materials is the ES hardware itself, which typically requires high voltage, electric isolation, and charged and flat deposition surfaces. In the past, fabrication of structures or materials with precisely determined mesoscale morphology has been accomplished through modification of electrode shape, use of multi-dimensional electrodes or pins, deposition onto weaving looms, hand held electrospinners that allow the user to guide deposition, or electric field manipulation by lensing elements or apertures. In this work, we demonstrate an ES system that contains multiple high voltage power supplies that are independently controlled. This system produces a novel electrostatic field that enables deposition of polymers in precise, mesoscale structures.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 63/177,131 filed on Apr. 20, 2021, the disclosure of which is hereby incorporated by reference in its entirety to provide continuity of disclosure.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Cooperative Agreement No. W911NF-15-2-0020 awarded by the Combat Capabilities Development Command Army Research Laboratory. The government has certain rights in the invention. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation herein.

THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT

Not applicable.

REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX

Not Applicable.

BACKGROUND OF THE INVENTION

Electrospinning (ES) fabrication was first observed in 1897 (Zeleny 1914) followed by a series of patents granted for textile applications. In 1969, a publication by Taylor (Taylor 1969) set in motion a body of research that utilized ES fabrication for a plethora of applications that sought to make polymer materials with micro- to nano-scale features that exhibited high surface-area-to-volume ratios. Since then, ES has been used to fabricate fuel cells, generators, and provide photocatalytic surfaces (Shi et al. 2015) as well as to prevent degradation of perovskite solar cell layers (Murphy, Andriolo, et al. 2016; Murphy, Andriolo, Sutton, Brockway, et al. 2017; Murphy, Andriolo, Sutton, Wyss, et al. 2017; Murphy, Ross, et al. 2016) and to pattern nanoscale polarizers via lithography (Beisel et al. 2016). Biomedical applications of electrospun materials include enzyme immobilization, sensors, tissue engineering, wound healing (Haider, Haider, and Kang 2018), and drug delivery (Andriolo et al. 2017, 2018; Hu et al. 2014). ES fiber materials have also been used for catalysis of nanomaterials that range in application from energy conversion to medicine and that exhibit desirable material properties such as high strength, high modulus (Zhang et al. 2014).

ES fabrication requires delivery of a solvent dissolved (Chronakis 2005; Doshi and Reneker 1995; Huang et al. 2003) or solid stick (Hutmacher and Dalton 2011) polymer by mechanical pump to a metallic spinneret held at a high voltage relative to a collection surface. Once polymer forms a bead just outside the tip of the spinneret, voltage initiated on the collection surface (electrode) causes surface charge buildup at the surface of the polymer bead. At critical value, the polymer bead is deformed into a cone (Taylor cone (Taylor 1969)). At the apex of the Taylor cone, a micro- or nano-scale polymer jet is pulled by electrostatic force toward the deposition surface, resulting in deposition of polymer fibers or beads. During flight, the polymer jet experiences a chaotic phase, whereby, solvent evaporation occurs (Reneker et al. 2000). The force required to initiate ES is emitted described by the following formula:

$\begin{matrix} {F_{es} = {\frac{\varepsilon_{r}\varepsilon_{0}}{2d^{2}}V^{2}A}} & \left( {{Eq}.1} \right) \end{matrix}$

where permittivity is represented by ε_r (relative) and ε_0 (in a vacuum), A is the area of the collection plate, V is the applied voltage, and d is the separation distance between spinneret and collection surface.

The breadth of materials that ES has enabled is far reaching and relevant in application from fundamental chemistry and synthesis to applied use to industry. The span of applicable uses for ES has led to iterations of ES equipment that accommodate implementation for fabrication of specialized materials. Melt ES, for example, allows the user to avoid the use of solvents during the process (Hutmacher and Dalton 2011). Other iterations involve alteration of the deposition surface to produce aligned structures that are beneficial for enhanced charge transport (Manafi and Badiee 2008; De Marco et al. 2008), production of polarized light emission (Pagliara et al. 2010; Zheng et al. 2007), improved absorption and photovoltaic properties (Xin et al. 2010; Xin, Kim, and Jenekhe 2008), and crystal properties beneficial for optoelectronics among other applications (Arinstein et al. 2007; Bellan and Craighead 2008; Kongkhlang et al. 2008; Stephens, Chase, and Rabolt 2004). Alignment is also relevant to the biomedical industry to provide a scaffold for directional cell growth (Yan et al. 2012) and guided cell differentiation (Breukers et al. 2010). Alignment of polymer fibers can be accomplished through the use of rotating collector drums (Mo and Weber 2004; Teo et al. 2005), parallel gap electrodes (Beisel et al. 2016; Li, Wang, and Xia 2003), or counter electrodes (Carnell et al. 2008). The electric field which provides electrostatic force for polymer deposition has also been manipulated to guide fiber deposition and material spot size (Beisel et al. 2014; Kooistra-Manning et al. 2019; Skinner et al. 2017). Passive methods for electric field manipulation include using copper rings as lensing elements to dampen chaotic motion (Deitzel et al. 2001) and use of aperture plates to reduce resulting fiber mat spot size (Skinner et al. 2015, 2017). Researchers have also accomplished miniaturization of the ES system and added configuration modifications that allow ES systems to be handheld and deposit onto any surface regardless of charge (Huston et al. 2019; Kooistra-Manning et al. 2019, 2020).

One major limitation to wide adoption of electrospun materials is the ES hardware itself, which typically requires high voltage, electric isolation, and charged and flat deposition surfaces. In the past, fabrication of structures or materials with precisely determined mesoscale morphology has been accomplished through modification of electrode shape, use of multi-dimensional electrodes or pins, deposition onto weaving looms, hand-held electrospinners that allow the user to guide deposition, or electric field manipulation by lensing elements or apertures. In this work, we demonstrate a Multiplex-ES system that is comprised of multiple high voltage power supplies that are independently controlled. This system produces a novel electrostatic field that enables deposition of polymers in defined locations anywhere within the ES device and production of mesoscale structures with precise micro to nanoscale features.

BRIEF SUMMARY OF THE INVENTION

The multiplex ES system was assembled inside an isolation box equipped with a momentary safety lever switch. At a first end of the isolation box, a holder was used to secure a spinneret in place. The spinneret is connected to a high voltage source or grounded via contact with a metal pogo pin. At a second end of the isolation box, a plurality of electrodes are secured in place and connected to independent high voltage power supplies by corresponding electrode metal pogo pins. Feedback from the plurality of power supplies was monitored by a Tektronix TDS 2004C oscilloscope. Manipulation of the high voltage electric field amongst the plurality of independently controlled electrodes allows for precise fiber deposition as the polymer fiber lays down on the deposition surface where the electric field is strongest.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1. Graphical representation of one configuration of the multiplex ES system. The multiplex ES system shown is comprised of four electrodes and a spinneret, each connected to an independently-controlled power supply. Independent control over each electrode enables guiding of polymer fibers or drops and deposition of defined and complex structures. Also shown in FIG. 1 is the electrical isolation box and grounded conductive tape used to prevent electric shock of the user, as well as components used to move the spinneret and/or electrodes to modify the electrostatic field during ES, ultimately resulting in polymer fibers, drops, or materials that have the desired configuration/s and properties.

FIG. 2. COMSOL Multiphysics® models representing the multiplex ES system. A. The generated model shows four electrodes placed equidistant from each other. During acquisition of the model, the high-voltage signal was supplied to electrode C as shown. B. The generated model shows the electrodes from with respect to the ES spinneret. All electrodes were placed equidistant from the spinneret in the multiplex ES system.

FIG. 3. A. Graphical representation of polymer deposition path and order of deposition during multiplex ES of woven fiber mats. During deposition, polymer deposition follows the electrode exhibiting the highest voltage (strongest electrostatic force). In instances 2, 4, and 6, the polymer jet jumps exterior to the center woven mat in order to avoid disrupting the woven material and enabling true weaving of the fibers to occur. B. SEM micrograph showing the precisely woven polymer fiber pattern graphically represented in A. C. is a digital light microscopy image shows the bulk three-dimensional configuration of the woven ES mats.

FIG. 4. Part A-C shows images of electrospun, polymer tori patterns fabricated using multiplex ES. Electrospun tori were removed from the system and placed in a light box prior to acquisition of images, which were thresholded with ImageJ. Parts D-F show corresponding images demonstrating the tori fiber mats after a threshold had been applied in ImageJ. Tori and images were collected in triplicate, and dimensions from these images were used to provide mathematical understanding of the fiber mat that would result when specific ES parameters were used.

DETAILED DESCRIPTION OF THE INVENTION

In general, use of the Multiplex-ES system 100 involves delivery of solvent dissolved or melted liquid polymer into a spinneret 120 via a pump. Once polymer is forced to the tip of the spinneret 120, electric voltage initiated in the electrodes 130 creates an electrostatic field within the Multiplex-ES system 100. The electrostatic force begins to stretch a polymer bead at the tip of the spinneret 120. The polymer then enters a chaotic region where polymer whips around and results in evaporation of solvent used to dissolve the polymer during preparation. Control over the voltage supplied to a plurality of electrodes guides deposition of polymer beads or fibers to a specific electrode, sequential electrodes, and/or in between electrodes to produce a precisely desired configuration of the deposited polymer beads or fibers. Using the device and methods described herein, precise weaving of fibers has been achieved as demonstrated in FIG. 3. Similarly, electrospun tori patterns have been achieved as shown in FIG. 4. One familiar with the art would recognize that multiple polymer specifications could be supplied to the system. In one embodiment, polymer supplied to the system is comprised of polymer dissolved in solvent. In another embodiment, polymer supplied to the system is melted. In yet another embodiment, polymer supplied to the system is comprised of semi conducting or conducting properties. In yet another embodiment, multiple polymers are supplied to the system through multiple spinnerets and delivery means where such delivery means would include pumps, syringes, and gravity fed.

The multiplex ES system 100 (FIG. 1) was assembled inside an isolation box 110 assembled from ⅛-in-thick acrylic sheets and equipped with a momentary safety switch 111. The safety switch 111 was integrated into the system via LabVIEW and programmed to force a connected data acquisition system (DAQ) output to 0 kV, should the door 112 of the isolation box 110 be opened. The DAQ system used for this work were USB-6008 and USB-6009 12-bit resolution National Instruments digital acquisition DAQs that regulated direct current (DC) and alternating current (AC) input signals. One familiar with the art would recognize that various data acquisition systems of various specifications could be utilized to practice the invention and data acquisition systems of alternative specifications are contemplated within the disclosure herein. Aluminum tape 113 was placed at the corners 114 of the isolation box 110 and grounded 115 to prevent buildup of electrical charge on the surfaces of the isolation box 110. In another embodiment, said isolation box is equipped with an encasement and ventilation system to allow for the introduction of a gas to the system.

At a first end 116 of the isolation box 110, a photopolymer resin spinneret holder 122 was used to hold a spinneret 120 in place. The spinneret is connected to a high voltage source via contact with a metal pogo pin 121. The spinneret may also be grounded. One familiar in the art would recognize that multiple spinnerets could be incorporated into the system to supply multiple polymers for ES and also said spinnerets could be comprised of various characteristics such as being coaxial or triaxial to produce fibers of differing characteristics. Electrode holders 131 were fabricated with a Formlabs Form2 405 nm SLA resin 3D printer. The photocatalytic resin was used in place of fused deposition modelling printing to prevent formation of trapped air spaces that can become charged by the power supplies and interrupt the electric field distribution in the multiplex ES system 100. In another embodiment, said spinneret holder 122 and electrode holders 131 are further comprised of mechanical or electrical components, which are utilized to move said spinneret 120 and electrodes 130 to further precisely alter the electric field shape, size, and/or strength.

At a second end 117 of the isolation box 110, four electrodes 130 a, 130 b, 130 c, and 130 d were held in place with said photopolymer resin electrode holders 131 to keep each electrode 130 a, 130 b, 130 c, and 130 d isolated and connected to each electrode's high voltage source by corresponding electrode metal pogo pins 132. The four deposition electrodes 130 a, 130 b, 130 c, and 130 d used were cut from 1/64 in thick Cu sheets. Each electrode 130 a, 130 b, 130 c, and 130 d was connected to an external National Instruments DAQ (data acquisition) system, which was connected to a corresponding independent electrostatic discharge electromagnetic compatible 20-kV/1-mA high-voltage power supply 140 a, 140 b, 140 c, and 140 d. The DAQ has both analog and digital input/outputs and enables both control over the high voltage power supplies, as well as provides a user with real-time voltage readings as the electrospinning process occurs. Control over the power supplies is fed from LabView through the DAQ and signal feedback is fed from the high voltage power supplies through the DAQ and can be read in LabView. One familiar with the art would recognize that various data acquisition systems and power supplies of various specifications could be utilized to practice the invention and data acquisition systems and power supplies of alternative specifications are contemplated within the disclosure herein. Feedback from said four power supplies was monitored by a Tektronix TDS 2004C oscilloscope.

The Multiplex-ES system 100 provides precise deposition of electrospun fibers 200 and mesoscale morphology control by manipulation of the high voltage electric field. The point of high voltage in the system 100 is moved from one electrode to another, thereby guiding fiber deposition as the polymer fiber lays down where the electric field is strongest. In FIG. 2, a COMSOL Multiphysics model is shown. In FIG. 2, the point of high voltage in the system 100 is electrode C 130 c. Therefore, in the theorized instance shown in FIG. 2, electrospun polymer would preferably deposit on electrode C 130 c. In the preferred embodiment, voltage supplied to each electrode is digitally controlled by a computer, which receives information from a or a plurality of DAQ's. Therefore by manipulating voltages to said electrodes simultaneously, fiber deposition can be precisely moved to any point within the four independently controlled electrodes 130 a, 130 b, 130 c, and 130 d. See FIGS. 3A and B. While the preferred means of controlling the voltages supplied to the electrodes is digital via a computer programming based on data from a DAQ, one familiar with the art would recognize that voltages could be controlled by analog or mechanical means with use of switches, knobs, buttons, and similar means. Similarly, while four electrodes were utilized in the herein disclosed embodiment, one familiar with the art would recognize that any plurality of electrodes could be utilized depending on the fiber deposition characteristics to be achieved.

Through control over electric field strength at specific locations within the system, fiber deposition is precisely controlled and can be utilized to fabricate highly accurate specified materials. FIG. 2 shows a COMSOL model used to visualize the multiplex ES system 100 when the highest voltage was supplied to one electrode (electrode C 130 c). In this instance (FIG. 2), fiber deposition would preferentially deposit on electrode C 130 c. If the highest voltage is directed to switch from a first electrode to a second electrode, fiber deposition follows the path between the first electrode and the second. See FIG. 3A. If two electrodes are supplied with similar high voltages, fiber deposition will occur between the electrodes rather than on them. Fiber deposition also occurs between electrodes as the polymer stream moves from one electrode or space to the next electrode or space within the system as shown on FIG. 3A.

One limiting factor to control over fiber deposition controlled by a computational system is response time of the power supplies to the computational input. In this system, the response time was improved up to 65% through manipulation of the impedance of the Multiplex-ES system by the addition of resistors to the high voltage circuit.

In other embodiments the Multiplex-ES system may be equipped with a high-resolution camera for real time viewing of the processes of the system. The Multiplex-ES system may be equipped with one or a plurality of lasers to aid in process control and feedback. The Multiplex-ES system may be equipped with one or a plurality of collimated light sources. The Multiplex-ES system may be equipped for electrical isolation. Interlock technology may be incorporated into the system. Safety shutoff technology may be utilized within the system. One familiar in the art would recognize the system could be configured so that polymer deposition is guided to a surface other than conductive electrodes or surfaces; polymer deposition is guided to a non-conductive surface; or where polymer deposition is guided to a conductive surface. Furthermore, the system is capable of scaling in size from handheld systems to large systems for large scale industrial applications. All are considered within the scope of the present disclosure.

It is contemplated that the device and methods described herein would include but not be limited to the following: fabrication of materials for medical applications; electrical applications; coating applications, chemistry applications; biological applications; fabrication of solid therapeutic materials, including medications; fabrication of powdered therapeutic materials, including medications; and fabrication of liquid therapeutic materials, including medications.

It is understood that the foregoing examples are merely illustrative of the present invention. Certain modifications of the disclosed device and/or methods may be made and still achieve the objectives of the invention. Such modifications are contemplated as within the scope of the claimed invention. 

What is claimed is:
 1. A Multiplex ES device comprising: A spinneret, which delivers a polymer; A plurality of electrodes, wherein said electrodes are each connected to an independent high voltage power supply; A means of controlling a voltage supplied to each of said electrodes by each said independent high voltage power supply to provide a high voltage electric field; wherein Manipulation of said high voltage electric field between said plurality of electrodes allows for precise control of deposition of a fiber from polymer delivered by said spinneret.
 2. The Multiplex ES device of claim 1, wherein said spinneret is grounded.
 3. The Multiplex ES device of claim 1, wherein said spinneret is connected to an independent high voltage power supply.
 4. The Multiplex ES device of claim 1, wherein said means of controlling a voltage supplied to each of said electrodes is digitally controlled by a computer.
 5. The Multiplex ES device of claim 1, wherein said means of controlling a voltage supplied to each of said electrodes is by analog or mechanical means with use of switches, knobs, buttons.
 6. The Multiplex ES device of claim 1, further comprising a or a plurality of data acquisition systems.
 7. The Multiplex ES device of claim 1, wherein said high voltage power supply provides between −30 kV and 30 kV.
 8. The Multiplex ES device of claim 1, further comprising one or a plurality of resistors added to a circuit of said high voltage power supply.
 9. The Multiplex ES device of claim 1, further comprising an isolation box, which contains said spinneret and said plurality of electrodes.
 10. The Multiplex ES device of claim 1, further comprising a plurality of spinnerets.
 11. The Multiplex ES device of claim 10, wherein multiple polymers are delivered to the system by said plurality of spinnerets.
 12. The Multiplex ES device of claim 1, further comprised of a spinneret holder, which secures said spinneret.
 13. The Multiplex ES device of claim 1, further comprised of a plurality of electrode holders, which secure said plurality of electrodes.
 14. The Multiplex ES device of claim 12, further comprised of mechanical or electrical components to enable movement of said spinneret.
 15. The Multiplex ES device of claim 13, further comprised of mechanical or electrical components to enable movement of said plurality of electrodes.
 16. The Multiplex ES device of claim 9, wherein said isolation box is further comprised of an encasement and venting system to allow for the introduction of a gas.
 17. The Multiplex ES device of claim 9, wherein said isolation box is further comprised of an encasement system to allow for polymer deposition under vacuum.
 18. The Multiplex ES device of claim 1, wherein said spinneret is coaxial.
 19. The Multiplex ES device of claim 1, wherein said spinneret is triaxial.
 20. A method of providing precisely controlled fiber deposition during electrospinning comprising the steps of: a. delivering polymer to the Multiplex ES device of claim 1; and b. manipulating said high voltage electric field between said plurality of electrodes to precisely control deposition of said fiber. 